Method of making spherules of a crystalline nuclear fuel carbide

ABSTRACT

METHOD OF PRODUCING NUCLEAR CARBIDE SPHERULES WHEREIN NUCLEAR PARTICLES OF CARBIDE OR OTHER COMPOSITION, ARE HEATED IN AN ISOLATING MEDIUM SUCH AS CARBON TO THE FUSION TEMPERATURE TO PRODUCE SPHERULES. WHEN STARTING WITH NON-CARBIDE PARTICLES ENOUGH CARBON IS INCLUDED TO FORM THE CARBIDE. THE SPHERULES ARE THEN GIVEN A PYROLYTIC CARBON COATING.

United States Patent 27,264 METHOD OF MAKING SPHERULES OF A CRYSTALLINENUCLEAR FUEL CARBIDE Harold G. Sowman, Maplewood, and James R. Johnson,

White Bear Lake, Minn., assignors to Minnesota Mining and ManufacturingCompany, St. Paul, Minn.

No Drawing. Original No. 3,163,609, dated Dec. 29, 1964, Ser. No.256,238, Feb. 5, 1963, which is a division of Ser. No. 96,081, Mar. 16,1961. Application for reissue Aug. 20, 1965, Ser. No. 484,785

Int. Cl. GZlc 21/02 U.S. Cl. 264-5 27 Claims Matter enclosed in heavybrackets II] appears in the original patent but forms no part of thisreissue Specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT OF THE DISCLOSURE Method of producing nuclear carbide spheruleswherein nuclear particles, of carbide or other composition, are heatedin an isolating medium such as carbon to the fusion temperature toproduce spherules. When starting with non-carbide particles enoughcarbon is included to form the carbide. The spherul'es are then given apyrolytic carbon coating.

This application is a division of our copending application Serial No.96,081, filed Mar. 16, 1961, which issued as U.S. Patent No. 3,129,188on April 14, 1964[.] and which was a continuation-in-part of ourcopending application Ser. No. 838,445, filed Sept. 8, 1959, nowabandoned.

This invention relates to spherules of refractory carbides and moreparticularly to spherules of boron, thorium and uranium carbides andprocesses for the production thereof.

It is known that when it is desired to design a nuclear reactor of largesize relative to the critical mass, it is necessary to provide a meansof controlling the neutron flux so that the reactor is notself-destructive. Thus, it is necessary to incorporate into the reactormaterials capable of absorbing and dissipating excess neutrons. It isparticularly desirable to incorporate as such materials substances whichbecome less efficient in absorption and dissipation of excess neutronsas operation of the reactor proceeds, since the accumulation offissioned materials normally lowers the efiiciency of the systemprogressively, until such point as a self-sustaining reaction is nolonger possible. Effectively removing the neutron absorbing materialsduring the life of such a reactor permits longer operation of the systembefore chemical reprocessing of the fuel elements is required.

An especially efiicient way of accomplishing the controlled butprogressive effective removal of neutron absorbers from atomic reactorsystems is by the use of destructible or burnable poisons in the reactorcore. By the term destructible or burnable poisons is meant substancescapable of absorbing neutrons, and which are thereby converted tonon-absorptive atoms. For example, boron as such or in the form of boroncarbide is converted to helium or lithium which are no longer effectiveneutron absorbers. To use boron carbide effectively for this purpose, itshould be distributed relatively homogeneously in the core or that partof the core in which it is desired to burn poison and fuelsimultaneously. The poison may be incorporated with the fuel or as aseparate element. Particles of these substances can be incorporated in ametal shell at a desired concentration to give the effective absorptioncapacity desired, for example, by thorough mixing with metallic powdersand following the technology of powder metallurgy to produce anintermediate compressed shape which is sintered to give a finalconfiguration. However, the use of ordinary particles of irregular shapebut of reasonably uniform individual size has been found to suffer fromthe disadvantage that during working of the metal further fragmentationtakes place, with the production of defects which permit diffusion ofthe helium produced by neutron capture, or an accumulation of the gasmay even result in a rupture of the metal container. Furthermore, if themetallic elements containing the burnable poison are mechanically workedin any way following their shaping by sintering, the irregular particlesbreak up and string out, thus causing internal porosity permitting rapidtransfer of fissioned gases or particles along the line of the defect.Spherical particles would greatly reduce these disadvantageous results.

A further problem in the production of atomic reactors is that ofproviding reactor fuel elements containing uranium carbide asfissionable material, in which the said fissionable material isdispersed in small particles throughout a pellet or matrix. Desirably,the particles are of substantially uniform size and shape in order togive the best possible results when incorporated into metallic matricesor employed in packed containers. While uranium carbide may of course bereduced by grinding or ball milling to particles of extremely small andfairly uniform size, although these may be of irregular shape, it isadvantageous to provide particles which are predictably regular.Spherical particles are by far the most desirable in this connection.Furthermore, because of the pyrophoric nature of uranium carbide,minimization of the surface area by the provision of spherular particlesis also very advantageous.

Fuel elements containing thorium carbide in combination with uraniumcarbide are useful for certain types of reactors. This combination ofnuclear fuel carbides appears to exist in the form of solid solutions inwhich the proportion of carbon is constant and the thorium-uranium ratiomay vary but together they are stoichiometrically equivalent to thecarbon. Compositions of this solid solution series are herein referredto a uranium (thorium) carbide.

It is an object of this invention to provide small substantiallyspherical solid particles of crystalline boron carbide, uranium carbideand uranium (thorium) carbide structures.

It is another object of this invention to provide a procass for theproduction of substantially spherical particles of high-meltingcarbides.

Still another object of the invention is to provide spherical particleswhich have surface coatings.

Other objects of the invention will become evident from the disclosureshereinafter made.

In accordance with the above and other objects of the invention, it hasbeen found that solid spherules of boron carbide and uranium (thorium)carbide can be formed by rapidly melting discrete particles of thesesubstances in admixture with a resilient inert isolating material of lowbulk density such as amorphous carbon, graphite, boron nitride, and thelike to form small molten spheres, and then cooling the mixture tosolidify the carbides in 3 spherical shape. The essential step appearsto be that of maintaining the discrete, isolated particles of boroncarbide or uranium (thorium) carbide in molten form for a time justsufficient to form spherules by operation of surface tension on themolten particle.

The uranium (thorium) carbide spherules according to the invention havethe average composition U Th C where m and n are numbers from to 1 andthe sum of m and n is 1. For such compositions there appears to be aprogressive series of solid solutions of increasing melting point fromUC, through U Th C to ThC preferably, m is less than 1 and greater thanzero.

By the term spherules as used herein, it is intended to designatesubstantially spherical structures having a diameter in the range ofabout microns up to about 125 mils, having a glossy surface formed fromthe molten or at least semi-molten state by the operation of surfacetension, while the interior of the said spherules has a crystallinestructure characteristic of the particular carbide system and issubstantially free from voids. In some instances, the spherules whenviewed under magnification appear to have a surface consisting of minutefacets. The presence of such minute surfaces of different radius ofcurvature is immaterial when the particle as a whole is substantiallyspherical. The spherules exhibit the property of rolling on slightinclines which is characteristic of spheres.

Surprisingly, while the spherules produced in the process of theinvention are crystalline as shown by X-ray diffraction studies, thesurfaces of the spherules are uniformly smooth and substantiallyspherical, presenting a highly polished appearance when inspected underthe microscope at moderate powers of magnification. The particles arefurthermore solid, by which is meant that they are substantially freefrom voids. If particles of uniform size are used as starting materials,the resulting spherules are also of substantially uniform diameter.However, the starting material may contain a wide range of sizes, andthe final product will in that case also have a wide variation indiameter. The spherules can of course be graded as to size by the use ofappropriate sieves.

While it has heretofore been known to produce small beads from glass orvitreous materials, so far as is known the art has not heretofore beenaware of any way in which spherules of hard crystalline substances, suchas boron carbide, can be made. It is highly unexpected to find thatcrystalline spherules of hard crystalline substances, such as boroncarbide, can be made. It is highly unexpected to find that crystallinespherules of boron carbide and ura nium (thorium) carbide can in fact beproduced without domination of the surface by crystal faces. Aside fromthe fact that spherules are formed from crystalline chemical compoundsas opposed to vitreous substances by the process of the invention, it isalso rather surprising to find that spherules of boron carbide areformed at all when carbon is used as an isolating medium, since it hasbeen found that molten boron carbide wets carbon surfaces and it mightbe expected that when fused it would interpenetrate the graphite orcarbon particles employed as an isolating medium or dissolve so muchcarbon as to form merely a boron carbide having high carbon content.Without being bound thereby, it may be hypothesized that the formationof spherules may result from a blanketing effect of the resilient oryielding finely divided carbon or other non-reactive low bulk densityisolating media employed. When coarse isolating powders are used, of theorder of particle size of the material which is to be made spherical,satisfactory spherules are not usually formed.

It is found that the carbon content of boron carbide spherules commonlyincreases slightly over that of the original particles onspheroidization by the process of the invention. This may be caused bysolution of some of the carbon in direct contact with the molten boroncarbide over a relatively larger area or along a particular crystalaxis. The resulting spherules usually contain boron and carbon in aratio of from about 3 to about 6 atoms of boron per atom of carbon. Itis known that boron carbides exist thus in a number of phases. Thephases which may be present can be determined by reference to the phasediagram of boron and carbon shown in Zhurnal Fizicheskoi Khimii, vol.32, pp. 2428, Octo er 1958.

Broadly speaking, the process of the invention is carried out byisolating small irregularly shaped discrete particles of a source ofuranium carbide, uranium (thorium) carbide or boron carbide by mixingthem with an isolating medium of low bulk density, placing the mixturein a suitable furnace in the presence of a non-reactive atmosphere,rapidly heating the mixture to a temperature sufiicient to formspherules owing to the surface tension forces acting on the molten orsemi-molten carbide, cooling the mixture, and removing the isolatingmaterial from the spherules.

The isolating medium used can be any substance not inter-reactive withthe carbides, which is of low bulk density, and which is not melted at,nor otherwise physically changed, at the temperatures used in theprocess. Examples of such materials are carbon and boron nitride infinely divided form.

Various compounds of uranium and boron, such as oxides, and the elementsthemselves may be employed as equivalents of the carbides, in that theyform the respective carbides at the temperatures used, when carbon iemployed as the isolating medium or in conjunction therewith.Furthermore, the corresponding compounds of thorium may be used withuranium compounds in any desired proportions, or in place thereof tofurnish uranium (thorium) carbide. Thus, sources of the uranium(thorium) and boron carbide (carbide-forming materials) containing themetal of the carbide to be formed from which the nuclear fuel carbide orpoison. carbide spherules of the present invention are produced areuranium (thorium) carbides and boron carbide [therselves] themselves,or, if carbon is employed, the metals, their oxides and the like. It ispossible that the reaction with carbon, which may be termed carburizing,which takes place when a source other than the final carbide is used asa starting material occurs more or less simultaneously with thespheroidization, and at a lower temperature than the melting point ofthe particular carbide. Without wishing to be bound by theory, it ispossible that in some cases spherules of lower melting materials, suchas boric oxide, are first formed, followed by rapid reaction with thecarbon to form the carbide. However, it is considered that this processis fully equivalent to the process in which the carbide actually existsin molten form.

It will be apparent that if a precursor for the uranium carbide, uranium(thorium) carbide, or boron carbide is used, such as the metals or theiroxides, it will be necessary to provide at least sufficient carbon inthe isolating mixture to react with the precursor in addition to theamount of isolant necessary to effect isolation of the particles.

While reference is made herein to boron metal, it is of course apparentthat boron is not a true metal in the generally accepted sense of theword, and it is to be under stood that the term metal in this connectionis used for convenience only as designating the element.

To avoid oxidation, it is preferred to heat the mixture in anon-reactive atmosphere, meaning by the term atmospheres which do notreact with any of the components of the mixture which is heated. It isknown that nitrogen may react with boron at the temperature employed inthe process to form boron nitride, which may in some instances beundesirable.

It is a rather surprising feature of the process that reaction to formspherules of carbides from sources other than uranium (thorium) or boroncarbides, e.g. the metals or oxides, proceeds so rapidly as to bevirtually complete in the relatively short time intervals employed. Thisis particularly striking because as suggested above, reaction may betaking place with spherular particles of the precursor which thenpresent a minimum surface for reaction.

While other finely divided isolating media may be employed, carbonpowder is conveniently available and is used herein to exemplifyisolating media in general as to the amounts and state of division whichcan be used. The carbon powder which is used need not be chemically purecarbon, but as used herein the term carbon is employed in the commontechnical sense and includes graphite, carbon black, lamp black,amorphous carbon, petroleum coke and the like low bulk density forms ofcarbon. Preferably, the particle size of the powdered carbon or otherisolating particles which is employed is at least about one order ofmagnitude smaller than that of the material to be converted tospherules, so that there are a large number of carbon particles for eachof the particles of carbide which is to be produced as a spherule. Inthis way the particles to be spheroidized are kept out of physicalcontact with each other. The effect is to provide a yielding, orresilient, supporting medium for the spherules as they form. Variousforms of finely divided carbon and the particle sizes thereof aredescribed in Industrial Carbon, Mantel], D. Van Nostrand Co., Inc., NewYork, 2nd ed., 1946.

Generally speaking, the [production] proportion of isolant to thecarbide or carbide precursor which is employed in the initial mixturemay be [carried] varied over a wide range in carrying out the invention.Thus, 100 parts by weight of isolant can be used for each 1 to 100 partsof starting material, although it should be understood that proportionsoutside of this range can also be used. While an unduly high proportionof carbon powder results in somewhat less economical operation, in thatthe heating of a relatively large mass of material is required beforefusion of the carbide particles takes place, this does not preventformation of spherules. On the other hand, too small a proportion ofcarbon prevents the mechanical separating action to the degree which isnecessary to obtain good spheres and apparently also affects theresiliency adversely. It will be apparent that when a precursorsubstance is used which forms the selected carbide on heating withcarbon, the amount of carbon is increased to compensate for the carbonwhich is consumed in the reaction, or an amount of carbon must be addedto the reation mixture, as the carbon must serve as a carbon source forthe chemical reaction to form the carbide. Preferably, a ratio of about5 to 20 parts of carbon by weight to 1 part by weight of carbide orcarbide precursor is employed, as providing better size control for thespherules.

The spherules can be made by a batch process. In this case, afterthorough mixing, to insure that the finely divided mixture of isolatingmedium and source of carbide is substantially homogeneous, it is placedin a suitable refractory container in a furnace, in an inert(nonreactive) atmosphere, and heated rapidly to a temperature in therange of about 2300 to 2700" C. As non-reactive atmospheres, argon,helium and the like may be mentioned as suitable, while nitrogen can beused but is somewhat less satisfactory owing to the possibility of theformation of nitrides. The chief function of the non-reactive atmosphereis to prevent the combustion of the carbon which would otherwise takeplace in the presence of oxygen at the temperatures employed, but theatmosphere used must not react with the carbide source or with theisolating medium. The temperature to be employed is that which is justsuflicient to melt the particular carbide which is selected, so that theforce of surface tension can draw each particle of the carbide into aspherical shape. The melting point of boron carbide is about 2450 C.while that of uranium carbide is about 2350 C. and of thorium carbideabout 2650" C. Although temperatures somewhat in excess of thesetemperatures can be employed, they are not necessary for successfulformation of the sphcrules and may result in some coalescing of thespherules to form larger spherules. While this may not be undesirable insome instances, it will be apparent that control of the diameter of thespherules will then be more difiicult.

Temperatures of the order required in the process, i.e. above about 2000C., are determined optically and therefore are not exact temperatures.Some variation in observed temperature from batch to batch mayconsequently be expected, as well as differences arising from personalvisual errors.

The heating of the mixture is continued only for so long as is necessaryto produce the spherules. The dwell time in the furnace, i.e. the lengthof time required to form spherules, is readily determined by empiricalmethads and depends on the mass of material being heated and thecapacity of the furnace. The mass is kept in the furnace just slightlylonger than required for the entire mass to reach the melting point ofthe car-bide, which is just sufficient to bring about transformation ofthe carbide particles to spherules. The mixture is then rapidly cooledto a temperature below the melting point of the particular carbideinvolved, for example, by removal of the mixture, while maintaining thenonreactive atmosphere, from the reaction zone to a zone in whichpositive cooling is accomplished. The rate of cooling is advantageouslymade high in the case of uranium (thorium) carbide in order to produceand maintain the fine crystalline structure of the dicarbide since underslow rates of cooling and in annealing there is some tendency forrecrystallization with the formation of (U, Th)C and carbon. The effectis observed, for example, in etched metallographie specimens which showfine striae after annealing which are lacking in quenched spherules.Furthermore, there are differences in the unit cell dimensions asdetermined by X-ray diffraction techniques which indicate that thequenched material is more dense. After cooling, the spherules areseparated from the remainder of the mixture, for example, by washing,screening, flotation techniques and the like.

Alternatively, a rotary kiln may be used for continuous larger volumecommercial production bearing in mind the absolute necessity of avoidingcritical masses. In this case, the mixture of isolant and startingmaterials is fed continuously into the upper end of a rotary kiln whichslopes downwardly from inlet to outlet. The inlet end of the kiln isheated by any convenient means to a temperature in the appropriate rangeand as the mixture progresses it rapidly becomes heated to causeformation of spherules. A non-reactive atmosphere is provided in thekiln. The kiln may be provided with a positively cooled zone, or themixture may be discharged into a stream of cold inert gas and allowed tofall into a receptacle. The scope of the kiln is adjusted to provide theproper dwell time for the temperature and heating unit employed. The useof a rotary kiln is less desirable since larger spherules are formed byagglomeration and size control is therefore more difficult.

In another method which can be used for making the spherules of theinvention, a bed of the selected isolating medium is spread upon arefractory pallet, such as a graphite block of appropriate dimensions.The isolating medium can be present in a relatively thin layer, as forexample about 10 to mils in thickness. The finely divided uranium orboron carbide is then sprinkled on this bed in a layer approximately oneparticle thick, and subjected to intense heat in a non-reactiveatmosphere until the carbide particles are drawn into spherules. Thepallet is thereupon removed from the heating zone and cooled.

It is commonly found that the isolating medium adheres to some extent tothe surfaces of the spherules, particularly when carbon is employed.Preferably, the sphcrules are treated with a bath containing adetergent, to remove the carbon or other materials as completely aspossible from their surfaces. However, where carbon is used, it will beapparent that for some applications it will not be necessary to removethe adherent carbon as completely as for others. If desired,substantially all of the carbon which may be adherent to the surface canbe removed therefrom by oxidation under controlled conditions, as forexample, by refluxing the particles with chromic acid solution, or byheating them in air at about 1000 C., in the case of boron carbide. Forremoval of particles of isolating material from uranium carbidespherules, if required, milling in a ball mill with an organic solventsuch as acetone, and with rubber-covered steel balls, removes theadherent material.

Cleaning of the spherules of the invention is also usefully accomplishedby employing ultrasonic vibrating devices. The process comprisessuspending the spherules in an inert liquid having a viscosity nogreater than that of water, preferably containing a small amount ofsurfactant, and subjecting the suspension to ultrasonic vibrations forabout 3 to minutes. The liquid is decanted and replaced with freshliquid repeatedly until suspended carbon is no longer evident therein.Uranium (thorium) carbide spherules are cleaned by this procedure usingnon-hydroxylated solvents since reaction with hydroxylated solvents suchas water may result in oxidation of the surfaces. Boron carbide iscleaned in the same manner, but water can be used.

Having thus described the invention in broad general terms it is nowmore specifically illustrated by examples which show the best modecontemplated of practicing the invention. In these examples all partsare by weight unless otherwise specified.

Example 1 A mixture of powdered boron carbide and graphite is preparedby mixing 6.75 parts of boron carbide of 140 to +325 mesh and 33.75parts of finely divided carbon (a furnace black or Thermatomic carbon),in a one pint glass jar in which are placed coiled iron wires. Mixing iseffected by rolling the jar on rollers of the type used for laboratoryball mills for minutes at about 112 r.p.m. The mixture is separated fromthe Wires avoiding manipulation which might cause separation of thesolid ingredients and the mixture is packed loosely in a carbon tube ofsuitable size loosely fitted at both ends with threaded graphite plugs.This is referred to as a boat.

The boat containing the batch is placed at the entrance of a carbon tubefurnace about 3 feet long and 3 inches in diameter approximately thecentral one-third portion of which is heated to a temperature of 2500 C.by passage of an electric current therethrough (resistance heating).Temperatures are determined optically. The furnace is flushed with argonto provide a substantially oxygen-free environment, i.e., anon-oxidizing atmosphere and to prevent oxidation of the carbon tube.Enough of the argon enters the boat to leave an inert atmosphere in theboat without special measures. After the boat has attained a bright redheat (about 700 C.) at the front of the furnace (about 2-3 minutes) itis moved into the central region of the furnace, held at 2500 C. andleft there for about 8 minutes. When first moved to this region there isa drop in temperature of the furnace, which is quickly restored and theboat attains the said temperature in about 3-5 minutes. The actual timerequired depends on the particular dimensions of the system. Theslightly longer residence time of 8 minutes permits the boron carbide tomelt and form molten spherules in the graphite matrix. The boat is thenmoved to the water cooled cold end of the furnace and permitted to coolrapidly to below a red heat (about 600 C.). This requires about 5minutes. The boat is then removed from the furnace and one plug removed.

When cooled sufficiently that the batch inside no longer appears toglow, the batch is poured into a large volume of dilute solution of adetergent such as an alkyl ether of polyethylene glycol (availablecommercially under the trademark Tergitol from Union Carbide and CarbonCorporation). The suspension is stirred to permit wetting of theparticles of carbon and spherules of boron carbide and poured through a325 mesh wire screen. The residue is washed repeatedly with clear waterand fresh detergent solution until the wash water no longer showsobservable discoloration due to carbon. The batch is then dried andfound to weigh slightly more than the boron carbide used due to a pickupof carbon partly as free occluded carbon and partly as carbon dissolvedin the boron carbide. The spherules vary in diameter from about 50 to200 microns, and are separated from a few percent of malformed particlesby rolling down an inclined plane or other method. Screening serves toclassify the spherules in various diameters if desired. Analysis forboron and carbon shows:

Percent Total B 67.74 Total C 34.23 Free C 12.09

The values for total B and C are presumably both slightly high. Theatomic ratio of boron to combined carbon is about 3.4:1 to about 3.5:1.The spherules show the characteristic X-ray diffraction patterns ofboron carbide. These spherules are hard and smooth and are suited forincorporation in the reactor designs as a burnable poison. Reference tothe above-cited phase diagram shows that this composition (24.6 percentC combined) corresponds to the 49' phase.

Example 2 When the preceding procedure is repeated, except that boron isused instead of the boron carbide, operating at a furnace temperature ofabout 2350 C. to about 2500" C., spherules of boron carbide areobtained. When the boron used as a starting material is of particle sizeto +325 mesh, the spherules obtained are approximately 40 to 200 micronsin diameter.

Example 3 The procedure of Example 1 is repeated, except that boron-richboron carbide, corresponding to B C and having a particle size of about100 to +400 mesh, is employed. A furnace temperature of about 2500" C.is used. Spherules of boron carbide, having the formula B C and about 37to microns in diameter, are obtained.

This procedure is repeated, except that boron-rich boron carbidecorresponding to the formula B C and having a particle size of l00 to+400 mesh is employed. The temperature used is the same as that used forthe B C particles.

Spherules of boron carbide corresponding to the approximate formula B CBC, and having a diameter ranging from about 37 to 150 microns, are thusobtained.

Example 4 The procedure of Example 1 is repeated, except that 7 parts ofpowdered boric acid are used. The furnace temperature employed is about2500 C. The dwell time in the furnace is about 10 minutes at furnacetemperature. Spherules of boron carbide, as shown by characteristicX-ray diffraction patterns, are obtained, having a range of diametersfrom 10 microns up to about 200 microns. The smaller spherules, havingdiameters ranging from about 10 to 30 microns, can be separated from thecarbon in the wash water by sedimentation and decantation.

Substantially the same results are obtained when 7 parts of 100 meshboric oxide (B 0 are substituted for the boric acid which ie employedabove. In this case, a furnace temperature of about 2450 C. is used, andspherules of boron carbide having a diameter range of from about 50 to200 microns are obtained. The spherules are shown to be boron carbide bytheir characteristic X- ray diffraction patterns.

Example The procedure of Example 1 is repeated, except that 13.5 partsof carbon are used. Spherules of boron carbide having an approximatediameter ranging from 50 microns to 200 microns are obtained, while anamount of the boron carbide agglomerates to larger diameters. Similarly,when the procedure of Example 1 is repeated, except that 67.5 parts ofcarbon are used, spherules having the same diameter range are obtained.The use of 337.5 parts of carbon for isolating 6.75 parts of boroncarbide likewise gives useful spherules, but the heating of this largemass of low bulk density requires an increase in dwell time in thefurnace and the separation of the spherules from the carbon becomessomewhat more difficult. The change in the proportions of carbonemployed does not substantially affect the amount of carbon which isfound upon the surfaces of the spherules.

Example 6 A larger batch of spherular crystalline boron carbide isprepared in a series of smaller batches as follows: Powdered boroncarbide of the approximate formula B C consisting of about 30 percent byweight of discrete particles of less than 100 mesh and more than 200mesh and the balance of discrete particles between 200- and 270 mesh iscarefully blended with ten parts by weight of finely divided carbon(furnace black, available under the trademark Thermax) in a twin-shellblender with an intensifier bar for five minutes. Portions of theblended mixture weighing approximately 110 grams are placed incylindrical graphite boats 11 inches long and 4 inches outside diameterwith M; inch thick walls and the boats are then successively placed in atube furnace in a flowing argon atmosphere at 2500 C. for ten minutesand then moved forward to a cooling zone to cool to around 600' C.Thereafter, the mixture is cooled to a convenient handling temperatureand the contents of the several boats are screened to remove carbon asabove by washing with Water containing a wetting agent and dried. Roughclassification to remove substantially non-spherular particles provides113 grams of substantially spherular crystalline boron carbide. Sieveanalysis shows that about half of the material is in the range of about35 to 50 microns in diameter mesh and most of the rest is in the rangeof about 50 to 150 microns in diameter. Some of the spherules fail topass a 70 mesh screen and approach 125 mils in diameter. The finermaterial contains an excess of carbon, probably unremoved isolatingcarbon.

Example 7 Ten parts of boron nitride of 325 mesh size (substantially allof the particles being about 10 microns in diameter) and 1 part of boroncarbide (13 C) of about -l00 to +270 mesh size are carefully mixed toinsure that the boron carbide particles are uniformly distributedthroughout the mixture. The mixture is then loosely packed into agraphite boat which is approximately 2%" in diameter and 7" long, andwith wall thickness of about A". The ends are loosely plugged withgraphite discs. The boat is placed in the entrance of a carbon tubefurnace as described in Example 1. The furnace is flushed with argon andthe argon atmosphere is maintained throughout the heating and coolingoperations. After the boat has attained a bright red heat, it is movedto the 2500 C. zone of the furnace and held there for about eightminutes. At this time the entire mass has become heated to a temperaturesufiicient to melt the boron carbide particles and form spherules. Theboat is then moved to the water-cooled cold end of the furnace andpermitted to cool rapidly. When it is no longer at red heat it isremoved from the funace and cooled until it can be handled conveniently.The boat is opened and the spherules which are formed are separated bywashing the boron nitride from the spherules, using copious quantitiesof a dilute aqueous detergent solution, followed with distilled water.

Spherules of 3 C ranging in diameter from about 60 to 150 microns arethus obtained.

Example 8 The procedure of Example 1 is repeated employing 1 part ofuranium carbide about -70 to +150 mesh in place of the boron carbide,with about 3 parts by weight of thermatomic carbon. Because of thepyrophoric nature of uranium carbide it must be handled cautiously. Thebatch is heated for 10 minutes at 2350 C. after first permitting it toheat at 1000 C. for 4 minutes. The spherular uranium carbide is isolatedfrom the batch after cooling as above by quickly washing the batchthrough a sieve which passes the carbon particles using water containinga small amount of detergent. Spherules of uranium carbide about -200microns in diameter are obtained. While water can be used for separatingthe spherules from the isolating medium when small batches areprocessed, non-hydroxylated organic solvents, such as benzene, arepreferred for use in processing large amounts of uranium carbidespherules to avoid the danger of decomposition. The spherules are driedin an argon atmosphere. The dry uranium carbide spherules are suitablefor dispersion-type reactor fuel elements. The carbon content of thecompound, in spherular form, is reduced from UC to predominantly UC byheating in dry hydrogen for 1 hour at 1300 C. This material is extremelypyrophoric and must be handled with care, under an inert atmosphere.

When powdered boron nitride is employed as an isolating medium forproducing uranium carbide spherules, there is a possibility ofcontamination of the resulting spherules of uranium carbide, as withboron. Consequently, when uranium carbide spherules of high purity arerequired, the isolating medium of choice is finely divided carbon.

When 7.5 parts of uranium dioxide, about 100 to 200 mesh particle size,are carefully blended with 22.5 parts of carbon black as in Example 1,and then heated in an argon atmosphere to 1000 C. for 4 minutes,followed by 10 minutes at 2350-2360 C., uranium carbide spherules areformed having diameters in the range of about 50 to microns. These areseparated from the isolating medium as before stated.

The preceding procedure is repeated, but using 7.5 parts of uraniumdioxide of about 100 to +200 mesh particle size, and 37.5 parts offinely divided carbon black and the batch, in an argon atmosphere, iskept at 1000 C. for 4 minutes and then maintained at about 2300-2375 C.for about 10 minutes. The batch is cooled and the spherules areseparated as before. Uranium carbide spherules ranging from about 50- to150 microns in diameter are obtained.

It is found that spherules are formed as above when the soaking, i.e.time of maintaining maximum temperature, is as short as ten minutes. Itis somewhat advantageous to permit soaking to proceed for about 20 to 30minutes in that there is better opportunity for spheroidization toproceed with better elimination of microscopic inclusions and voids. Anannealing at a lower temperature, e.g. 2200" C., for two hours alsoserves to perfect the shape and density of the spherules.

When uranium dioxide is employed as the starting material it isadvantageous to assure proper reaction by first mixing the uraniumdioxide intimately with about 2.0 to 4.0 moles of carbon per mole ofuranium dioxide and a small amount of an organic binder. A mixture of100 parts of U0 (less than 50 micron size), 9 parts of fine carbon and 4parts of polyvinyl alcohol is ball milled for 2 hours to a homogeneousmixture. The dark gray mixture is dried and granulated to a size ofabout 250 to 350 microns and the granulated material is blended with anapproximately equal weight of thermatomic carbon and spherulized asabove by heating to 2525 C. for 30 min- 11 utes. The mixture is cooled,separated from the bulk of the supporting carbon by ultrasonic washingwith benzene or isooctane with frequent decantation of the supernatantliquid and addition of fresh solvent. The cleaned particles no longermake a smudge when rubbed on uncalendered paper and are approximately150 to 250 microns in diameter. Metallographic examination of thespherules in section shows good crystalline structure and substantiallycircular outlines.

For certain nuclear reactor uses it is desirable that the spherules ofuranium carbide or uranium (thorium) carbide or the oxides, be sheathedwith a tough cladding which is substantially impervious to fissionproducts and yet non-absorptive of neutrons. Such coatings can beprovided by plating the spherules with metals, by coating withappropriate ceramic powders together with a binder followed by firing,by decomposition of gases to deposit hard carbon on the spherules, orthe like.

It is found that a particularly useful coating is provided by a layer ofcarbon deposited by pyrolysis of a hydrocarbon under certain conditions.Since the layer of pyrolytic carbon thus produced shows propertiesdifferent from those of graphite or ordinary soft carbon it isconvenient to refer to it by a distinctive term and it is herein termedpyrocarbon.

It is known that pyrocarbon coatings can be applied to relatively largearticles, such as ceramic tubes and the like. However, the problem ofapplying a pyrocarbon coating to the spherules of the invention israther more difficult than the coating of tubular ceramic bodies sincesequential mechanical movement and exposure as used heretofore isentirely unfeasible with such small objects. It has furthermore beenfound that somewhat higher temperatures are operative. This may beassociated with a failure of the smooth substantially spherical surfacesof the spherules of the invention to provide any catalytic effect on thedecomposition of the methane employed as a source of carbon, as comparedwith articles such as ceramic tubes which presumably have a rough andgrainy surface.

It is found that pyrocarbon is deposited on the spherules as a toughcoating by the pyrolysis of methane, carbon monoxide or the like at atemperature of about 1300" to 1700 C. Preferably a temperature in therange of about 1300 C. to 1450 C. is used. While some deposition ofpyrocarbon occurs on static spherules, the preferred procedure is tomaintain the spherules in motion to achieve a more uniform coating oneach spherule. This is achieved by slowly dropping the spherules througha heated zone, by vibrating a suitable receptacle containing thespherules in a heated zone, by tumbling the spherules by rotation of theheated zone, or by fiuidizing a bed of the spherules using an inert gas,for example, helium, at the temperatures noted above.

Example 9 About 50 grams of spherules of UC; (about 200 to 250 micronsin diameter) prepared as described in the first part of Example 8 aboveare placed in a rotating graphite drum about 3 inches in diameter and 4inches long having gas inlet and outlet connections rotatably mounted ina 3 /2 inch diameter quartz tube. A coil surrounding the tube is coupedwith a 15 kw. kc. generator so that the crucible is heated by induction.A stream of argon containing 10 percent by volume of methane is passedthrough the tube to displace air and the crucible and contents areheated to about 1300 to 1350 C. (determined by an optical pyrometer).Heating and rotation of the crucible are continued for about 1 hour andthe flow of methane is cut off and the crucible allowed to cool in thestream of argon. It is removed and the spherules are found to be coatedwith a tough hard layer of pyrocarbon about 30 microns thick.

Example 10 A mixture of 1 part of granules of pressed uranium oxide andcarbon in 9:1 ratio and about 300 micron diameter and 2 parts by weightof Thermatomic carbon (finely divided, furnace black) is made by placingthe ingredients in a twin shell blender and mixing thoroughly. A batchof desired amounts is packed loosely in a carbon tube (boat) which isloosely fitted at both ends with threaded graphite plugs and which is ofsuitable size to fit into the furnace used. The tube with the batch ispermitted to heat at 1000 C. for four minutes. It is then moved to thecentral zone and there heated (fired) at 2550 C. Heating at thistemperature for about 30 minutes produces substantially void-freespherules. After firing the batch-containing tube is moved to the end ofthe furnace, which is cooled with water and permitted to cool rapidly tobelow red heat. About five minutes are required for this cooling,whereupon the tube is removed from the furnace, one plug removed and thebatch is poured into an argon allutriation separator in which the finelydivided carbon is blown away from the larger spherules of uraniumcarbide with argon, in a continuous winnowing operation, and thespherules are very rapidly cooled. In this operation, nitrogen can alsobe employed as the inert gas, a conical vessel fitted for introductionof gas at the small lower end is provided with a foraminous (gaspermeable) support near the same end on which the charge is placed.Passage of gas carries the fine particles away while the largerparticles remain behind. The spherules which are obtained are aboutl00200 microns in diameter. They are preferably stored in an inertatmosphere such as dry argon. Spherules thus obtained can be metalplated to provide a useful coating. Thus, for example, a quantity of UCspherules are placed in a copper dish which is made the cathode in anelectroplating cell. A sheet of copper is used as the anode. The cell isfilled with an electrolyte having the following composition:

H 50 (conc.) ml 37.0 CuSO, g 20.0 Water to make 1 liter.

The anode is located a short distance above the cathode. The spherulesin the cathode are continuously agitated with a stream of argon-flowingat a rate of about 2-3 cu. ft./hour. A current of about 0.45 ampere at1.5 volts is passed for ten minutes. At the end of that time thespherules are found to have a copper coating. This can be increased inthickness by continuing the plating process.

Example 11 This example illustrates the formation of uranium (thiorium)carbide spherules of the invention.

A mixture of 50 parts of 325 mesh thorium oxide, 20 parts of uraniumoxide similar to that employed in Example 8 and 7 parts of furnace black(Thermax") are milled in a ball mill having alumina balls with 230 partsof water containing 0.5 part of Tergitol TMN alkyl ether of polyethyleneglycol, a surfactant, and 2.0 pa rts of polyvinyl alcohol as a binder toproduce a creamy magma which is cast into a dish and dried at 75 C. inan oven for 24 hours to give a friable cake. (This provides about 2.3mols of carbon per mol of uranium and thorium oxides.) The cake isground to a powder in a mortar and pestle and screened through a meshscreen to give a fine powdery material. This procedure is employed toobtain a mixture as homogeneous as possible since it is desired that thespherules made each have the same composition. The powdery material isformed into pellets by pressing at 23,000 p.s.i. and is again pulverizedin a mortar and pestle and screened and classified into particle sizesof to 300 microns.

One part of the 150 to 300 microns size granules is combined and blendedwith 2 parts of carbon (thermatomic) for 30 minutes in V blender. Theentire mixture (comprising the 150 to 300 micron size particles in a bedof carbon powder) is placed in a graphite boat in a carbon tube furnaceas described above and fired in a flowing argon atmosphere. Heating upto 2550 C. requires hour and a temperature of about 2550 to 2580 C. isthen maintained for 30 minutes whereby the uranium (thorium) oxides arecarburized to produce the corresponding dicarbides and the dicarbideparticles are melted suflicienrly to spheroidt'ze them. The boat is thenremoved to the cool end of the furnace and allowed to cool, maintainingthe stream of argon for one hour. The carbon including the spherules ofuranium (thorium) carbide is suspended in dry isooctane and agitated inan ultrasonic cleaner for about three minutes. The spherules settleimmediately the agitation stops and the thick carbon suspension isdecanted. This process is repeated about 3 times until the supernatantliquid is clear. The resultant uranium (thorium) carbide spherules, inwhich the ratio of thorium to uranium is about to 2 (approximate formulaU Th Cz), are further classified to remove a few irregular particles andscreened. They are substantially spherical particles of about 100 to 200micron diameter.

The spherules are coated with pyrocarbon in the apparatus and by theprocedure described in Example 9 for uranium carbide spherules. Thespherules are thus heated in argon containing 90 percent by volume ofmethane for 2 hours, to provide a pyrocarbon coating on the spherules.The resultant spherules have a 60 micron thick coating of pyrocarbon.They are suitable for use in graphite matrix fuel elements.

By repeating the above procedure employing various proportions ofthiorium oxide, spherules are formed having any desired ratio of thoriumto uranium. When uranium is omitted the spherules formed are of thoriumcarbide. Thus, for example, by repeating the above procedure using 1part of thiorium dioxide and nine parts of uranium dioxide, 0 dicarbideof the approximate formula of the resulting material is U Th C and when9 parts of thorium dioxide and 2 parts of uranium dioxide are used, theapproximate formula is U 'l'h C As the proportion of thorium increasesthe temperature of firing is increased so that in the absence of uraniuma temperature at least above about 2655 C., the reported melting pointof ThC and ranging up to about 2900 C., is employed. Thus, by repeatingthe procedure set forth hereinabove, but using 70 parts of 325 meshthorium oxide, there is produced spherular thorium dicarbide of 150-300microns diameter. This may also be coated with pyrocarbon if desired.

What is claimed is:

1. The method for producing solid spherules of a crystalline material ofthe class consisting of uranium carbide, thorium carbide and uranium(thorium) carbide which comprises the transformation to spherical shapeof small irregularly shaped discrete particles of a material of thegroup consisting of uranium, thorium, uranium carbide, uranium (thorium)carbide and compounds of uranium and thorium which react with carbon onheating to form uranium carbide, thorium carbide and uranium (thorium)carbide, said discrete particles being isolated from one another by anisolating medium characterized by low bulk density, resiliency, lack ofundesirable reactivity and, when said small irregularly shaped particlesare other than a carbide, by the presence of at least a sufiicientamount of carbon to produce the corresponding canbide in addition to theisolating medium, by subjecting said particles and their matrix ofisolating medium to rapid heating in a non-reactive atmosphere for asufficient time and at a suflicient intensity to effect fusion of thediscrete small particles in the isolating medium thereby subjecting saiddiscrete particles to the action of surface tension forces inherent insaid particles and rendering them substantially spherical, cooling theresultant discrete spherular particles and their matrix of isolatingmedium and separating said discrete spherular particles from saidisolating medium.

2. The method for producing crystalline carbide spherules whichcomprises isolating small irregularly shaped particles of a member ofthe group consisting of uranium carbide, thorium carbide and uranium(thorium) carbide with an isolating medium of the class consisting ofboron nitride and carbon, rapidly melting the isolated particles in thesaid isolating medium and in a non-reactive atmosphere until they areformed into discrete spherules under surface tension, cooling thespherules, and separating the spherules from the isolating medium.

3. The method for producing spherules of crystalline uranium carbide,which comprises isolating particles of a uranium-containing startingmaterial which yields uranium carbide upon heating with carbon with anisolating medium of low bulk density of the group consisting of carbonand boron nitride containing carbon in amount at least sufficient toreact with the starting material to form uranium carbide in addition tothe isolating medium when a starting material other than uranium carbideis used, rapidly heating the said particles and isolating medium in anon-reactive atmosphere to melt them and to form spherules of uraniumcarbide by the action of surface tension forces inherent in saidparticles, cooling the spherules, and separating the spherules from theisolating medium.

4. The method for producing spherules of crystalline uranium (thorium)carbide, which comprises isolating particles of a uranium andthorium-containing starting material which yields uranium (thorium)carbide upon heating with carbon with an isolating medium of low bulkdensity of the group consisting of carbon and boron nitride containingcarbon in amount at least sufficient to react with the starting materialto form uranium carbide in addition to the isolating medium when astarting material other than uranium (thorium) carbide is used, rapidlyheating the said particles and isolating medium in a nonreactiveatmosphere to melt them and to form spherules of uranium (thorium)carbide by the action of surface tension forces inherent in saidparticles, cooling the spherules, and separating the spherules from theisolating medium.

5. In the method for producing crystalline carbide spherules, the stepwhich comprises maintaining discrete small particles of a member of thegroup consisting of crystalline uranium carbide, crystalline thoriumcarbide and crystalline uranium (thorium) carbide, in admixture with anisolating medium comprising a member of the group consisting of boronnitride and carbon, in molten form in a non-reactive atmosphere for aperiod of time just sufficient to form carbide spherules by operation ofsurface tension on the molten particles and cooling the molten particlesto a temperature below their melting point.

6. In the method for producing spherules of crystalline uranium carbide,the step which comprises maintaining discrete, finely divided particlesof uranium carbide, in admixture with a low bulk density isolatingmedium of the class consisting of boron nitride and carbon, in moltenform in a non-reactive atmosphere for a period of time just sufiicientto form spherules of uranium carbide by operation of surface tension onthe molten particles and cooling the said spherules to a temperaturebelow their melting point.

7. In the method for producing spherules of crystalline uranium(thorium) carbide, the step which comprises maintaining discrete, finelydivided particles of uranium (thorium) carbide, in admixture with a lowbulk density isolating medium of the class consisting of boron nitrideand carbon, in molten form in a non-reactive atmosphere for a period oftime just sufficient to form spherules of uranium (thorium) carbide byoperation of surface tension on the molten particles and cooling thesaid spherules to a temperature below their melting point.

8. A method of making crystalline uranium carbide spherules, whichcomprises isolating 1 part by weight of irregular particles of uraniumcarbide in admixture with finely divided low bulk density carbon inamount of about 1 to 100 parts by Weight, melting the said irregularparticles in a non-reactive atmosphere for a period of time justsufficient to bring about formation of uranium carbide spherules;cooling the spherules, and separating the uranium carbide spherules fromthe carbon.

9. A method of making crystalline uranium (thorium) carbide spherules,which comprises isolating 1 part by weight of irregular particles ofuranium (thorium) carbide in admixture with finely divided low bulkdensity carbon in amount of about 1 to 100 parts by weight, melting thesaid irregular particles in a non-reactive atmosphcre for a period oftime just sufiicient to bring about formation of uranium (thorium)carbide spherules; cooling the spherules, and separating the uranium(thorium) carbide spherules from the carbon.

10. The method for producing crystalline carbide spherules whichcomprises melting small discrete irregular particles of a material ofthe class consisting of uranium, thorium, uranium carbide, thoriumcanbide, uranium (thorium) carbide and compounds of thorium and ofuranium which react with carbon on heating to form thorium carbide,uranium carbide and uranium (thorium) carbide, in contact with a finelydivided solid isolating medium, the said isolating medium consistingessentially of carbon when the said particles are other than a carbide,in a non-reactive atmosphere, to form spherules of the selected carbideunder the influence of surface tension forces inherent in moltenparticles; and cooling the said spherules to a temperature below theirmelting point to solidify the said carbide in spherical form.

11. The method for producing crystalline carbide spherules of the classconsisting of uranium carbide, thorium carbide and uranium (thorium)carbide which comprises transforming to liquid spherules small discreteirregularly shaped particles of a material of the class consisting ofuranium, thorium, thorium carbide, uranium carbide, uranium (thorium)carbide and compounds of uranium and of thorium which react with carbonon heating to form uranium carbide, thorium carbide and uranium(thorium) carbide by rapidly heating the said particles in anon-reactive atmosphere, and in contact with a finely divided solidisolating medium, the said isolating medium consisting essentially ofcarbon when the said particles are other than a carbide, cooling thespherules to solidify them, and separating the solid spherules from theisolating materials.

12. The method for producing crystalline spherules of the classconsisting of uranium carbide, thorium carbide and uranium (thorium)carbide, which comprises forming molten isolated substantially sphericalparticles of the selected carbide under the operation of surface tensionforces inherent in said molten particles in contact with a finelydivided solid isolating medium and in an inert atmosphere, substantiallyimmediately cooling the spherules thus formed, and separating thespherules from the isolating materials.

13. The method for producing spherules of a member of the groupconsisting of crystalline uranium carbide, crystalline thorium carbideand crystalline uranium (thorium) carbide, which comprises mixing amaterial of the group consisting of uranium, thorium, uranium carbide,thorium carbide, uranium (thorium) carbide, and compounds of uranium andof thorium which react on heating to form uranium carbide, thoriumcarbide and uranium (thorium) carbide, with a finely divided isolatingmedium of the group consisting of boron nitride and carbon, the saidisolating medium containing at least enough carbon to produce thecorresponding carbide when a material other than a carbide is present,heating the mixture in a non-reactive atmosphere for a period of timesufficient to form crystalline carbide spherules under the operation ofsurface tension forces inherent in molten particles, cooling themixture, and separating the carbide spherules from the remainder of themixture.

14. A method of preparing dense, spheroidized, unagglomerated nuclearfuel carbide particles, which method comprises the steps of mixingtogether particulate nuclear fuel oxide, particulate carbon in aconcentration sufiicient to form carbide with substantially all of saidnuclear fuel, and carbonizable binder, making a magma of said mixture ina solvent for said binder, drying said magma and particulating saidmixture, uniformly dispersing the resultant fuel particles of from about300 to 500 micron size in a concentration of finely divided graphitesufiicient to maintain said fuel particles out of physical contact witheach other in said bed, disposing said dispersion in a reaction zone andheating said fuel particles in said zone in a nonreactive atmosphere tocarburizing temperature for said nuclear fuel, maintaining said fuelparticles at said temperature until carburization is substantiallycompleted and thereafter increasing the temperatures of said particlesto above the melting point of the nuclear fuel carbides formed in situin said fuel particles, maintaining said fuel particles at saidtemperature until substantially all of said particles have melted andhave spheroidlzed, thereafter cooling said particles to solidify thesame, and separating said solidified particles from non-adheringgraphite, whereby dense, hard, spheroidized, unagglomerated nuclear fuelcarbide particles are provided.

15. The method of claim 14 wherein said nuclear fuel oxide comprisesuranium oxide.

16. The method of claim 14 wherein said nuclear fuel oxide comprisesthorium oxide.

17. The method of claim 15 wherein said nuclear fuel oxide also includesthorium oxide, and wherein said binder comprises an organic binder.

18. A method of preparing dense, spheroidized, unagglomerated nuclearfuel carbide particles, which method comprises the steps of mixingtogether particulate nuclear fuel material and carbonizable binder, saidnuclear fuel containing a metal from the class consisting of uranium andthorium, the mixture containing an amount of carbon at least equivalentto the amount of nuclear fuel metal in said nuclear fuel, particulatingsaid mixture and uniformly dispersing the resultant fuel particles offrom about 10 micron to mil size in finely divided graphite so as tomaintain said fuel particles out of physical contact with each other insaid graphite, disposing said dispersion in a reaction zone and heatingsaid fuel particles in said zone in a substantially oxygen-freeenvironment to above the melting point of the nuclear fuel carbides ofsaid fuel particles, maintaining said particles at said temperatureuntil substantially all of said particles have melted and havespheroidized, and thereafter cooling said particles to solidify thesame, whereby dense, hard, spheroidized, unagglomerated nuclear fuelcarbide particles are provided.

19. The method for producing solid spherules of a crystalline metallicnuclear fuel carbide which comprises the transformation to sphericalshape of small irregularly shaped discrete particles of acarbide-forming material containing the metal of the nuclear fuelcarbide to be formed, said discrete particles being isolated from oneanother by an isolating medium characterized by low bulk density,resiliency, lack of undesirable reactivity and, when said smallirregularly shaped particles are other than a carbide, by the presenceof at least a sufficient amount of carbon to produce the correspondingcarbide in addition to the isolating medium, by subjecting saidparticles and their matrix of isolating medium to rapid heating in anon-reactive atmosphere for a sufiicient time and at a sufficientintensity to efiect formation of the carbide by reaction with carbonwhen the particles are other than a carbide and to bring about fusion ofthe discrete small particles in the isolating medium thereby subjectingsaid discrete particles to the action of surface tension forces inherentin said particles and rendering them substantially spherical, andcooling the resultant discrete spherular particles to solidify them.

20. The method of fabricating a nuclear fuel metal carbide particlewhich comprises mixing together a carbide-forming material containingthe metal of the desired nuclear fuel carbide, organic binder and anamount of carbon for reaction with said carbide-forming material inaddition to the carbon present in said organic binder and the carbon tobe used as the isolating bed, converting said mixture into particles anddispersing said particles in a bed of finely divided carbon, heating insaid bed to above the carburizing temperature of said particles, andsubse quently heating said particles in said bed to above the meltingpoint thereof so as to spheroidize the same, cooling said particles tobelow the melting point thereof, separating said particles from saidcarbon bed and heating said particles in a gaseous hydrocarbonatmosphere above the pyrolytic decomposition temperature for saidhydrocarbon to form a tough, hard protective adherent outer coating ofpyrolytic carbon about said particle.

21. The method for producing solid spherules of a crystalline materialof the class consisting of uranium carbide, thorium carbide and uranium(thorium) carbide which comprises the transformation to spherical shapeof small irregularly shaped discrete particles of a material of thegroup consisting of uranium, throium, uranium carbide, uranium (thorium)carbide and compounds of uranium and thorium which react with carbon onheating to form uranium carbide, thorium carbide and uranium (thorium)carbide, said discrete particles being isolated from one another by anisolating medium characterized by low bulk density, resiliency, lack ofundesirable reactivity and, when said small irregularly shaped particlesare other than a carbide, by the presence of at least a sufiicientamount of carbon to produce the corresponding carbide in addition to theisolating medium, by subjecting said particles and their matrix ofisolating medium to heating in a non-reactive atmosphere for asufficient time and at a sufficient intensity to efiect fusion of thediscrete small particles in the isolating medium thereby subjecting saiddiscrete particles to the action of surface tension forces inherent insaid particles and rendering them substantially spherical, cooling theresultant discrete spherular particles and their matrix of isolatingmedium, separating said discrete spherular particles from said isolatingmedium, and heating said particles in a gaseous hydrocarbon atmosphereto above the pyrolytic decomposition temperature for said hydrocarbon toform a protective adherent outer coating of pyrolytic carbon about saidparticles.

22, The method for producing solid spherules of a crystalline metallicnuclear fuel carbide which comprises the transformation to sphericalshape of small irregularly shaped discrete particles of acarbide-forming material containing the metal of the nuclear fuelcarbide to be formed said discrete particles being isolated from oneanother by an isolating medium characterized by low bulk density,resiliency, lack of undesirable reactivity and, when said irregularlyshaped particles are other than a carbide, by the presence of at least asufficient amount of carbon to produce the corresponding carbide inaddition to the isolating medium, by subjecting said particles and theirmatrix of isolating medium to heating in a non-reactive atmosphere for asuflicient time and at a sufficient intensity to efiect formation of thecarbide by reaction with carbon when the particles are other than acarbide and to bring about fusion of the discrete small particles in theisolating medium thereby subjecting said discrete particles to theaction of surface tension forces inherent in said particles andrendering them substantially spherical, cooling the resultant discretespherular particles to solidify them, and heating said particles in agaseous hydrocarbon atmosphere to above the pyroltic decompositiontemperature for said hydrocarbon to form a protective adherent 18 outercoating of pyrolytic carbon about said particles.

23. The method of fabricating a nuclear fuel metal carbide particlewhich comprises mixing together a carbideforming material containingmetal of the nuclear fuel carbide to be formed, carbont'zable binder andabout 2 to 4 mols of carbon per mol of said carbide-forming material,converting said mixture into particles and dispersing said particles ina bed of finely divided carbon, heating in said bed to above thecarburizing temperature of said particles, and continuing heating saidparticles in said bed to above the melting point thereof so as tospheroidize the same, cooling said particles to below the melting pointthereof, separating said particles from said carbon bed and heating saidparticles in a gaseous hydrocarbon atmosphere above the pyrolyticdecomposition temperature for said hydrocarbon to form a protectiveadherent outer coating of pyrolytic carbon about said particles.

24. A method of preparing dense, spheroidized unagglomerated [nuclearfuel carbide particles, which method comprises the steps of mixingtogether material including particulate nuclear fuel and carbonizablebinder, the mixture containing an amount of carbon at least equivalentto the amount of nuclear fuel metal in said nuclear fuel, particulatingsaid mixture and uniformly dispersing the resultant particles of fromabout 300-500 micron size in a sufficient amount of finely dividedgraphite so as to maintain said fuel particles out of physical contactwith each other in said graphite, disposing said dispersion in areaction zone, and heating said fuel particles in said zone in anon-reactive atmosphere to above the melting point of nuclear fuelcarbides, maintaining said fuel particles at said temperature until saidfuel particles have melted and have spheroidized, and thereafter coolingsaid fuel particles to solidify the same, whereby dense, hard,spheroidized, unagglomerized nuclear fuel carbide particles areprovided.

25. The method of fabricating spherules of a crystalline material of theclass consisting of uranium carbide, thorium carbide and uranium(thorium) carbide which comprises mixing together oxide of uranium,thorium or uranium and thorium, carbonizable binder, and about 2 to 4mols of carbon per mol of said oxide, converting said mixture intoparticles and dispersing said particles in a bed of finely dividedcarbon, heating in said bed to above the carburizing temperature of saidparticles, and continuing heating said particles in said bed to abovethe melting point thereof so as to spheroidize the same, cooling saidparticles to below the melting point thereof, separating said particlesfrom said carbon bed and heating said particles in a gaseous hydrocarbonatmosphere above the pyrolytic decomposition temperature for saidhydrocarbon to form a protective adherent outer coating of pyrolyticcarbon about said particles.

26. The method of claim I wherein the irregularly shaped discreteparticles are oxides and there is at least sufficient carbon in additionto the isolating medium to produce the corresponding carbide.

27. The method for producing solid spherules of a crystalline metallicnuclear fuel carbide which comprises the transformation to sphericalshape of small irregularly shaped discrete particles of acarbide-forming material containing the metal of the nuclear fuelcarbide to be formed, said metal being selected from the classconsisting of uranium and thorium, said discrete particles being with insuch a size range as to produce spherular particles of diameters from I0 microns to mils, and being isolated from one another by an isolatingmedium characterized by low bulk density, resiliency and lack ofundesirable reactivity and, when said irregularly shaped particles areother than a carbide, including at least a suflicient amount of carbonto produce the corresponding carbide in addition to the isolatingmedium, by subjecting said particles and their matrix of isolatingmeduim under conditions which avoid formation of a critical mass toheating in a nonreactive atmosphere to a temperature in the range ofabout 2300 to 2700 C. for a snfi'icient time and at a sufiicientintensity to efiect formation of the carbide by reaction with carbonwhen the particles are other than a carbide and to bring about fusion ofthe discrete small particles in the isolating medium thereby subjectingsaid discrete particles to the action of surface tension forces inherentin said particles and rendering them substantially spherical, coolingthe resultant discrete spherular particles to solidify them, and heatingsaid particles in a gaseous hydrocarbon atmosphere to above thepyrolytic decomposition temperature for said hydrocarbon to form aprotective adherent outer coating of pyrolytic carbon about saidparticles.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

OTHER REFERENCES AEC Report ORNL-l633, December 1953, p. 1. Progress inNuclear Energy, vol. 5, 1956, pp. 435-436.

10 ABC Report KAPIrM-ATM1, November 1958, pp.

AEC Report BMI-l357, June 1959, pp. 12, 52 and 53.

REUBEN EPSTEIN, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. Reissue27,26 4 Dated Januar ll, 1972 Inventofl Harold G. JSowman and James R.Johnson It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 12, "preferably" should read --Preferably-- Colunm 5,line 2, "present" should read -presents-- Column 6, line 50, "scope"should read --slope-- Column 7, line 55, "bright" should be in italics.

Column 13, line 32, "thiorium" should read -thorium-- Column 17, line27, "throium" should read --thorium-- Signed and sealed this 22nd day ofAugust 1972.

(SEAL) Attest:

EDWARD PLFLETCHER, JR ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents FORM P -1 i USCOMM-DC 60375 Ma 3 U 5 GOVERNMENT PRINTINGOFFCE: I953 0-365-334 Dedication Reissue N0. 27,264.Har0ld G. Smrmum,Maplewood, and James R. Johnson, White Bear Lake, Minn. METHOD OF MAKINGSPHERULES OF A CRYSTALLINE NUCLEAR FUEL CARBIDE. Patent dated Jan. 12,1972. Dedication filed May 8, 1973, by the assignee, the United Statesof America. Hereby dedicates t0 the Public of the United States theportion of the term of the patent subsequent to Feb. 12, 1973.

[Ofliaz'al Gazette November 2'7, [973.]

